(1) Field of the Invention
The invention relates to the treatment of wastewater and more particularly, to a septic system design with true siphon dosing and effluent filtration with backwash.
(2) Background of the Invention and Description of Previous Art
It is common knowledge among sanitary engineers that to prolong the life of septic fields it is necessary to clean or filter the effluent from the septic tank and to rapidly discharge a measured quantity (dose) of filtered effluent to flood the septic fields. The dose volume is normally about 70% of the septic field volume. This procedure extends the life of the septic field by distributing the effluent, and more importantly, the residual solids contained in the effluent, over the entire field rather than only near the entrance to the field where they will accumulate and eventually clog the first few feet of the septic field thus rendering a portion of the field's capacity to percolate effluent useless. Once this deterioration starts it will overload the remaining functioning portion of the field, which will lead to a total failure of the field in due time. The replacement of a failed septic system is very costly and messy operation.
Before describing the prior state of the art in this field it would be useful to keep in mind that septic tanks are buried underground to prevent freezing or for esthetic reasons. There is, in general, six inches to a foot or two of earth on top of the tank. To remove the tank covers for pumping out the tank contents or to inspect for malfunctioning components of the system, earth over the tank covers must be dug out to gain access. Access to the septic tank is not easy and is generally beyond the aptitude of most building owners. Typically a functioning septic tank should be pumped out once every two or three years.
The following patents were examined to ascertain the prior state of the art in this field.
Hanford, William E.,U.S. Pat. No. 4,040,962Ball, Harold L.,U.S. Pat. No. 4,439,323Gavin, Norman,U.S. Pat. No. 4,838,731Daniels, Byron C.,U.S. Pat. No. 5,198,113Graves, Jan D.,U.S. Pat. No. 5,207,896Richard, James G.,U.S. Pat. No. 5,290,434Ball, Eric S.,U.S. Pat. No. 5,492,635Stuth, William L.,U.S. Pat. No. 5,690,824Wilkins, Charles A.,U.S. Pat. No. 6,231,764
Methods and apparatus for improving the performance of septic system as described in the above mentioned patents have found limited use due to one or more of the following drawbacks.                1. High initial investment.        2. Cannot be retrofitted in existing installations.        3. Costly maintenance and operation.        4. Complexity and bulk.        5. High elevation differential requirements.        6. Not true siphons.        7. No dosing provision.        8. Insufficient or no filtration.        9. No provision to indicate the need for backwash or filter replacement.        10. Need for external power (electric pumps).        11. Need for frequent refill of chemicals.        12. Not intended for Septic Systems.        
It is important to recognize the enormity of the job a residential septic system must perform over its intended life time which is typically between 20 or more years. A typical single family consumes approximately 1000 gallons of water every day. Over a 25 year period the septic system processes over 9 million gallons or 76 million pounds of effluent. The quality of domestic waste dumped into the septic system varies greatly with the lifestyle of each family, particularly if a food disposal unit is utilized to grind kitchen waste and send it to the septic tank. The amount of sludge and flotsam removed by frequent tank pump outs will also vary over a wide range.
The suspended solids are of most concern because they clog up the septic fields. Daniels, '113 utilizes open cell polymeric foam to filter the effluent from the septic tank. No information is provided regarding the particle size of the solids, which will pass through the filter medium. However it is easy to guess from the general description that the particle size will be fairly small. Although the filter will remove most of the suspended solids from the effluent, frequent replacement of the filter element is necessary. This requires shoveling away the earth to expose the cover, removing the cover and replacing the filter, replacing the cover and the earth, not a welcome task with the ground frozen solid in winter. The reference also requires an electric pump to move the filtered effluent from the dosing chamber to the septic septic fields, thereby requiring the provision of electric service at the tank.
Ball, '635 describes a series of multiple size filters the smallest of which has an opening of ⅛th of an inch. Here too an electric pump is required, and has no backwash system. A ⅛th of an inch opening in the filter will pass 3,175-micron particles. Graves, '896 describes a multistage filtration process, which aims to filter particles as small as 1000 microns. However this process is dependent upon chlorination, aerobic agitation, and optional de-chlorination, thus requiring electrical power and chemicals. When the filters clog, the unit must be removed and cleaned. As with the previous reference this requires exposing and opening the tank to clean or replace the filter, again a substantial undertaking. No dosing mechanism is provided so, with the exception of the filter, the septic system has the site limitations of a simple gravity fed system.
Filtration systems are generally categorized by the particle size, which will pass through them. Particle size is generally measured in microns. A Micron is one millionth of a meter or 40 millionth of an inch. For reference the high quality drinking water filters block particles larger than 5 microns from passing through them. Some coarse drinking water filters would pass 30 micron particles. With respect to filtration in a septic system, to pass solids of over 3,000 microns is tantamount to no filtration at all. A great majority of suspended particles in a septic tank are much smaller; therefore much finer filter media are necessary to clean the effluent significantly. It becomes clear why Ball, '635 cites that the filter requires cleaning only as often as the container (the septic tank) requires pumping to remove accumulated sludge.
It is difficult to establish the suspended particle size distribution of the effluent, because each family's life style is different. Assuming a linear distribution of particle size the following table will illustrate the importance of filtration medium.
TABLE IQuantity of suspended solids in 76 million lbs (38,000 Tons) ofeffluent processed over a lifetime of 25 years.Percent (by Weight)Quantity1.0%(10,000 ppm.)380 Tons0.1% (1,000 ppm.) 38 Tons0.001%   (100 ppm.) 3.8 Tons0.0001%     (10 ppm.)760 lbs.
Even at the lower concentrations there is sufficient quantity and volume of suspended solids, which if not removed by filtration would plug up any septic field. Clearly, the importance of good effluent filtration cannot be overemphasized.
By comparison this invention filters particles as small as 100 to 200 microns by using a super fine filter. A 100 mesh screen (10,000 holes per square inch) will filter 180 micron or larger particles and a 150 mesh screen (22,500 holes per square inch) which will filter 100 micron or larger particles, and the effluent will be almost as clean with respect to suspended particles as the domestic water supply, a great benefit for the life of the septic field.
The present invention shows the use of a precision and true siphon to dose and filter the liquid extracted from the central clear zone of a septic tank. The unit is contained in a housing mounted and ported on the discharge side of a septic tank. Liquid flows into the housing from the bottom thereof, passes through a fine mesh basket strainer or filter, and rises in the housing until enough is collected to start the siphon. Then the siphon is initiated through the lifting of a float and the proper dose is delivered to a distribution box. After delivery of the dose volume the siphon is broken, and a predetermined short time thereafter a remote timer triggers a solenoid valve, which sends pressurized domestic water to backwash the filter for a predetermined time after which the system becomes ready for the next dosing cycle.
Referring now to FIG. 11, there is shown a typical septic system 120 comprising a septic tank 122, a drainpipe 124, and a distribution box 126. The inflow 127 to the septic system 120 is delivered through a pipe 121 emanating from a building or house (not shown) and received at the inlet of the septic tank 122. A septic field towards which the outflow 128 from the distribution box 126 is directed is not shown.
The total elevation difference 130 is defined as the difference in elevation between the bottom of the inlet pipe 121 and the bottom of the lowest level in the distribution box 126. The total elevation difference 130 can be further broken down to the sum of the septic tank drop 132, the pitch drop 134, and the distribution box drop 136.
The selection of an effluent delivery system i.e. a gravity siphon, or a pump system depends on the total elevation difference 130. In most health jurisdictions the minimum required difference 132 between the inlet and outlet of the septic tank is about three inches, but in some cases it can be as much as six inches or more. The pitch drop 134 depends upon the distance 135 between the septic tank and the distribution box. Most health departments require that a pitch or gradient of 1 in 100 or about ⅛ of an inch per foot of drainpipe length be maintained. The distribution box drop 136 is normally about one inch. The pitch drop 134 dictates the choice of an effluent disposal system as follows:                a. If the pitch drop 134 is insufficient to maintain the required pitch or if the distribution box is at a higher elevation than the liquid level in the septic tank, then it becomes necessary to install a pumping system.        b. If the pitch drop 134 is just enough to maintain a pitch of 1 in 100, then a simple gravity system is the only choice.        c. If the pitch drop 134 is large enough to meet the incremental elevation differential requirements, then a classical Bell Siphon (not shown) or the so-called siphon systems (some of which are included in the list of patents cited i.e. Ball, '323 and Richard, '434) on the market can be used.        
Referring to FIG. 12, these systems 140 require a dosing chamber 148 downstream of the septic tank 142, which holds the entire dose volume. Depending upon the dose volume, which governs the dimensions of the dosing chamber 148, the incremental siphon drop 147 can be anywhere from 6 to 18 inches on top of the pitch and distribution box drop 149. The pitch drop here is measured from the input to the drainpipe 144 near the bottom of dosing chamber 148. The dose volume is denoted in the figure by 146. The septic tank drop 143 is measured between the bottoms of the entry and exit pipes of the tank 142, and plays no role in the performance of the above mentioned so called siphon systems.
Neither, Ball, '323 nor Richard, '434 are true siphons, because the effluent is always under a positive hydrostatic head, and there is no vacuum anywhere in the drainpipe. A true siphon is defined as a continuous tube (siphon tube) that allows liquid to drain, without requiring pumping assistance, from a reservoir at a higher elevation to a point at a lower elevation, where the tube passes through an intermediate point that is higher than the reservoir. The up flow from the reservoir is driven by the pressure difference created by the vacuum formed by the siphon process at the highest point of the siphon tube.